testtest.py
testmodulesbase.py
testmodulesblah.py

File "modules/blah.py", line 1, in <module>
    from .base import BaseModule
ImportError: attempted relative import with no known parent package

#!/usr/bin/env python
import importlib
class Test():
    def __init__(self):
        spec = importlib.util.spec_from_file_location("blah", "modules/blah.py")
        mod = importlib.util.module_from_spec(spec)
        spec.loader.exec_module(mod)
def main():
    t = Test()
if __name__ == '__main__':
    main()

class BaseModule():
    modulename = "base"
    def __init__(self,name):
        print("Initializing module %s" % (self.modulename))

from .base import BaseModule
class BlahModule(BaseModule):
    modulename = "blah"